Edge Capping Of 2D-Mxene Sheets With Polyanionic Salts To Mitigate Oxidation In Aqueous Colloidal Suspensions

ABSTRACT

Provided are methods of stabilizing MXene compositions using polyanionic salts so as to reduce the oxidation of the MXenes. Also provided are stabilized MXene compositions.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of Ser. No. 17/617,082, “EdgeCapping Of 2D-Mxene Sheets With Polyanionic Salts To Mitigate OxidationIn Aqueous Colloidal Suspensions” (filed Jun. 12, 2020) (now allowed),which is the National Stage Application of International PatentApplication No. PCT/US2020/037487 (filed Jun. 12, 2020), which claimspriority to and the benefit of U.S. patent application No. 62/860,970,“Edge Capping Of 2D-Mxene Sheets With Polyanionic Salts To MitigateOxidation In Aqueous Colloidal Suspensions” (filed Jun. 13, 2019), theentireties of which applications are incorporated herein by referencefor any and all purposes.

GOVERNMENT RIGHTS

This invention was made with government support under Grant No.DMR-1740795 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

TECHNICAL FIELD

This disclosure is directed to methods of stabilizing MXene compositionsand the resulting stabilized MXene compositions.

BACKGROUND

Since the discovery of graphene, several new 2D materials likephosphorene, transition metal dichalcogenides (TMDs), layered transitionmetal carbides/nitrides (MXenes), and the like have been extensivelystudied for their interesting electronic and optical properties. Butunlike graphene, most of these 2D materials are unstable under ambientconditions and degrade over time due to oxidation. This severely hamperstheir potential to be used in practical devices and several strategieslike their encapsulation by deposition of air stable oxides, polymercoatings, covalent functionalization, heteroatom atom doping, etc., havebeen shown to reduce or completely stop the oxidation process.

The MAX phases are a large family of machinable, atomically layeredternary carbides, nitrides or carbonitrides. They are defined by ageneral chemical formula M_(n+1)AX_(n) (1≤n≤3), where M stands for earlytransition metal, A for group 13 or 14 elements and X represents Cand/or N. MXenes, first discovered in 2011, are obtained by etching outthe A layer from MAX phases most commonly by using and fluoride ioncontaining acid.

MXenes have a 2D morphology, with a general chemical formula ofM_(n+1)X_(n)T_(z) (1≤n≤3), or M_(1.33)CT_(z), where T stands for varioussurface terminations like —F, —OH and/or —O. Since the initial discoveryof Ti₃C₂T_(z), which is the by far the most studied MXene, more than 30new ones have been discovered. MXenes have shown applicability innumerous applications, including (without limitation) EMI shielding,energy storage, gas sensing, water purification, catalyst for hydrogenevolution reaction, and others.

Early reports on Ti₃C₂T_(z) aqueous colloidal suspensions, demonstratedthat oxidation occurred faster in aerated water than non-aerated water.This was taken to imply that dissolved oxygen in water was responsiblefor the oxidation. Zhang et al. were the first to systematically studythe oxidation of Ti₃C₂T_(z) colloids and showed that the suspensionswere more stable under argon (Ar). They also showed that flakes withsmaller lateral dimensions oxidized faster. Another important conclusionfrom their work was that Ti₃C₂T_(z) oxidation starts at the flakes'edges and proceeded inwards. Huang and Mochalin showed that Ti₃C₂T_(z)suspensions are more resistant to oxidation in isopropanol (IPA) than inwater which, in turn, implies that it is not just dissolved oxygen thatcauses oxidation but also the water molecules themselves.

One way to prevent oxidation is to store MXenes in organic solvents. Butthe fact that MXenes form less stable suspensions in organic solventscompared to water presents a challenge to this approach. Another issueis the relatively lower MXene concentration colloids present in IPA ascompared to water. Chae et al. found that storing MXene colloids atsub-zero temperatures (−18° C. and −80° C.) significantly increasestheir shelf life due to slowed oxidation kinetics, but oxidation wasfound to at even 5° C., thus making this approach relatively energyintensive. Accordingly, there is a need in the art for methods ofmitigating oxidation in MXenes, as well as a need for MXene compositionsthat exhibit reduced oxidation.

SUMMARY

In meeting the described needs, the present disclosure is directed tostabilizing MXene compositions. Herein is disclosed the finding thataddition of inorganic sodium salts of polyphosphates, polyborates, andpolysilicates to MXene colloid can slow the oxidation process. Thesepolyphosphates salts were chosen because they have been shown to adsorbat edges of clay sheets.

In one aspect, the present disclosure provides methods of stabilizingMXene compositions against oxidation, comprising: contacting a MXenecomposition with a polyanionic salt, the contacting being performed soas to give rise to a corresponding polyanionic salt-stabilized MXenecomposition.

Also provided are polyanionic salt-stabilized MXene compositions derivedor derivable from a method of the present disclosure.

Further provided are compositions, comprising: a MXene compositionhaving at least one layer having first and second surfaces and an edge;and at least one polyanion associated with the edge.

Additionally provided are electronic devices, comprising a compositionaccording to the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunctionwith the included drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale.

FIG. 1 provides a schematic of end-capping MXene materials.

FIGS. 2A-2C provide XRD patterns—log scale—of FIG. 2A 0.3BTi (red, top),0.5B Ti (black, bottom) FIG. 2B 0.3PTi (red, top), 0.5PTi (black,bottom) FIG. 2C (red, top), 0.5SiTi (black, bottom)

FIGS. 3A-3B provide SEM micrographs of FIG. 3A FV, FIG. 3B 0.1PVsamples. Scale bar=1 μm.

FIGS. 4A-4C provides XPS spectra of FIG. 4A 0.3BTi (black)), 0.5BTi(red) FIG. 4B 0.3PTi (black), 0.5PTi (red) FIG. 4C 0.3SiTi (black),0.5SiTi (red). The 0Ti (green) and FT (blue) spectra are added to eachgraph as reference.

FIG. 5 provides EELS spectra at 3 spots labeled in FIG. 6B: S1 (vacuum),S2 (edge) and S3 (surface).

FIGS. 6A-6B provide XRD patterns of, FIG. 6A 0.1PTi (blue), 0.1SiTi(green), 0.1BTi (red) and 0PTi (black). FIG. 6B 0.1PV (black), FV (red)samples. The patterns are shifted vertically for clarity. Inset showsdigital photographs of i) 0.1PV (left) sample, and ii) 0V sample (right)after 3 weeks.

FIGS. 7A-7D provide SEM micrographs of FIG. 7A 0Ti, FIG. 7B 0.1PTi,)0.1BTi, FIG. 7D samples. Scale bar=1 μm.

FIGS. 8A-8D provide TEM micrographs of FIG. 8A 0Ti, FIG. 8B 0.1PTi, FIG.8C 0.1BTi, FIG. 8D 0.1SiTi samples. Scale bar=0.2 μm.

FIGS. 9A-9B provide XPS spectra of, from top to bottom, FIG. 9A of0.1PTi (blue), 0.1SiTi (green), 0.1BTi (red), 0Ti (black), FTi (black).Gray band at 460 eV represents the Ti⁺⁴ oxidation state BE. FIG. 9B0.1PV (red, top) and FV (black, bottom) samples. Gray bands representthe V⁺⁵ oxidation state BE.

FIGS. 10A-10C provide TEM EELS analysis of Ti₃C₂T_(z) flake exposed tophosphate polyanions. FIG. 10A Normalized intensities of P, Ti and CEELS signals in going from vacuum towards the edge of the MXene flakealong LP1. The arrow gradients from blue to red, where blue colorrepresents the area over vacuum and red the area over the MXene flake.The arrow marks correspond to the positions from where the signal wasobtained. Similar arrows are marked on TEM image shown in b for easycomparison, FIG. TEM micrograph of flake used for EELS analysis. Scalebar=100 nm. FIG. 10C Same as a, but along LP2.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

MXenes have shown promise in many applications such as energy storage,catalysis, EMI shielding, among many others. However, MXene oxidation inaqueous colloidal suspensions when stored in water at ambient conditionsremains a challenge. Herein we show that by simply capping the edges ofindividual MXene flakes—herein exemplified as Ti₃C₂T_(z) and V₂CT_(z)—bypolyanions such as polyphosphates, polysilicates and polyborates it ispossible to quite significantly reduce their propensity for oxidationeven in aerated water for weeks. This breakthrough is consistent withthe realization that the edges of MXene sheets were positively charged.It is thus the first example of selectively functionalizing the edgesdifferently from the MXene sheet surfaces.

While exemplified for these two MXene compositions, the methods employedhere (and resulting compositions) are believed to extend to other MXenecompositions. MXene compositions are also sometimes described in termsof the phrase “MX-enes” or “MX-ene compositions.” MXenes may bedescribed as two-dimensional transition metal carbides, nitrides, orcarbonitrides comprising at least one layer having first and secondsurfaces, each layer described by a formula M_(n+1)M_(m)T_(x) andcomprising:

-   -   a substantially two-dimensional array of crystal cells,    -   each crystal cell having an empirical formula of M_(n+1)X_(n),        such that each X is positioned within an octahedral array of M,    -   wherein M is at least one Group IIIB, IVB, VB, or VIB metal,    -   wherein each X is C, N, or a combination thereof;    -   n=1, 2, or 3; and wherein    -   T_(x) represents surface termination groups.

These so-called MXene compositions have been described in U.S. Pat. No.9,193,595 and Application PCT/US2015/051588, filed Sep. 23, 2015, eachof which is incorporated by reference herein in its entirety at leastfor its teaching of these compositions, their (electrical) properties,and their methods of making. That is, any such composition described inthis Patent is considered as applicable for use in the presentapplications and methods and within the scope of the present invention.For the sake of completeness, M can be at least one of Sc, Y, Lu, Ti,Zr, Hf, V, Nb, Ta, Cr, Mo, or W. In certain embodiments in this class, Mis at least one Group IVB, Group VB, or Group VIB metal, preferably Ti,Mo, Nb, V, or Ta. Certain of these compositions include those having oneor more empirical formula wherein M_(n+1)X_(n) comprises Sc₂C, Ti₂C,V₂C, Cr₂C, Cr₂N, Zr₂C, Nb₂C, Hf₂C, Ti₃C₂, V₃C₂, Ta₃C₂, Ti₄C₃, V₄C₃,Ta₄C₃, Sc₂N, Ti₂N, V₂N, Cr₂N, Cr₂N, Zr₂N, Nb₂N, Hf₂C, Ti₃N₂, V₃C₂,Ta₃C₂, Ti₄N₃, V₄C₃, Ta₄N₃ or a combination or mixture thereof. Inparticular embodiments, the M_(n+1)X_(n) structure comprises Ti₃C₂,Ti₂C, Ta₄C₃ or (V_(1/2)Cr_(1/2))₃C₃. In some embodiments, M is Ti or Ta,and n is 1, 2, or 3, for example having an empirical formula Ti₃C₂ orTi₂C. In some of these embodiments, at least one of said surfaces ofeach layer has surface terminations comprising hydroxide, oxide,sub-oxide, or a combination thereof. In certain preferred embodiments,the MXene composition is described by a formula M_(n+1)X_(n)T_(x), whereM_(n+1)X_(n) are Ti₂CT_(x), Mo₂TiC₂T_(x), Ti₃C₂T_(x), or a combinationthereof, and T_(x) is as described herein. Those embodiments wherein Mis Ti, and n is 1 or 2, preferably 2, are especially preferred.

In other embodiments, the articles of manufacture and methods usecompositions, wherein the two-dimensional transition metal carbide,nitrides, or carbonytride comprises a composition having at least onelayer having first and second surfaces, each layer comprising:

-   -   a substantially two-dimensional array of crystal cells,    -   each crystal cell having an empirical formula of        M′₂M″_(n)X_(n+1), such that each X is positioned within an        octahedral array of M′ and M″, and where M″_(n) are present as        individual two-dimensional array of atoms intercalated        (sandwiched) between a pair of two-dimensional arrays of M′        atoms,    -   wherein M′ and M″ are different Group IBB, IVB, VB, or VIB        metals (especially where M′ and M″ are Ti, V, Nb, Ta, Cr, Mo, or        a combination thereof),    -   wherein each X is C, N, or a combination thereof, preferably C;        and    -   n=1 or 2.

These compositions are described in greater detail in ApplicationPCT/US2016/028354, filed Apr. 20, 2016, which is incorporated byreference herein in its entirety at least for its teaching of thesecompositions and their methods of making. For the sake of completeness,in some embodiments, M′ is Mo, and M″ is Nb, Ta, Ti, or V, or acombination thereof. In other embodiments, n is 2, M′ is Mo, Ti, V, or acombination thereof, and M″ is Cr, Nb, Ta, Ti, or V, or a combinationthereof. In still further embodiments, the empirical formulaM′₂M″_(n)X_(n+1) comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃,Cr₂TiC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, or V₂TiC₂,preferably Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, or Mo₂NbC₂, or their nitride orcarbonitride analogs. In still other embodiments, M′₂M″_(n)X_(n+1)comprises Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Cr₂Ti₂C₃, Cr₂V₂C₃,Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃, V₂Ta₂C₃, V₂Nb₂C₃, orV₂Ti₂C₃, preferably Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Ti₂Nb₂C₃,Ti₂Ta₂C₃, or V₂Ta₂C₃, or their nitride or carbonitride analogs.

Each of these compositions having empirical crystalline formulaeM_(n+1)X_(n) or M′₂M″_(n)X_(n+1) are described in terms of comprising atleast one layer having first and second surfaces, each layer comprisinga substantially two-dimensional array of crystal cells. In someembodiments, these compositions comprise layers of individualtwo-dimensional cells. In other embodiments, the compositions comprise aplurality of stacked layers. Additionally, in some embodiments, at leastone of said surfaces of each layer has surface terminations (optionallydesignated “T_(s)” or “T_(x)”) comprising alkoxide, carboxylate, halide,hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,thiol, or a combination thereof. In some embodiments, at least one ofsaid surfaces of each layer has surface terminations comprisingalkoxide, fluoride, hydroxide, oxide, sub-oxide, or a combinationthereof. In still other embodiments, both surfaces of each layer havesaid surface terminations comprising alkoxide, fluoride, hydroxide,oxide, sub-oxide, or a combination thereof. As used herein the terms“sub-oxide,” “sub-nitride,” or “sub-sulfide” is intended to connote acomposition containing an amount reflecting a sub-stoichiometric or amixed oxidation state of the M metal at the surface of oxide, nitride,or sulfide, respectively. For example, various forms of titania areknown to exist as TiO_(x), where x can be less than 2. Accordingly, thesurfaces of the present invention may also contain oxides, nitrides, orsulfides in similar sub-stoichiometric or mixed oxidation state amounts.

In the present disclosure, these MXenes may comprise simple individuallayers, a plurality of stacked layers, or a combination thereof. Eachlayer may independently comprise surfaces functionalized by any of thesurface coating features described herein (e.g., as in alkoxide,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, thiol, or a combination thereof) or may be alsopartially or completely functionalized by polymers, either on thesurface of individual layers, for example, where the two-dimensionalcompositions are embedded within a polymer matrix, or the polymers maybe intercalated between layers to form structural composites, or both.

General Terms

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. Finally, while anembodiment may be described as part of a series of steps or part of amore general structure, each said step may also be considered anindependent embodiment in itself, combinable with others.

The transitional terms “comprising,” “consisting essentially of,” and“consisting” are intended to connote their generally in acceptedmeanings in the patent vernacular; that is, (i) “comprising,” which issynonymous with “including,” “containing,” or “characterized by,” isinclusive or open-ended and does not exclude additional, unrecitedelements or method steps; (ii) “consisting of” excludes any element,step, or ingredient not specified in the claim; and (iii) “consistingessentially of” limits the scope of a claim to the specified materialsor steps and those that do not materially affect the basic and novelcharacteristic(s) of the claimed invention. Embodiments described interms of the phrase “comprising” (or its equivalents), also provide, asembodiments, those which are independently described in terms of“consisting of” and “consisting essentially of” For those compositionembodiments provided in terms of “consisting essentially of,” the basicand novel characteristic(s) is the ability to provide the describedeffect associated with the description as described herein or asexplicitly specified.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list, and everycombination of that list, is a separate embodiment. For example, a listof embodiments presented as “A, B, or C” is to be interpreted asincluding the embodiments, “A,” “B,” “C,” “A or B,” “A or C,” “B or C,”or “A, B, or C.”

Throughout this specification, words are to be afforded their normalmeaning, as would be understood by those skilled in the relevant art.However, so as to avoid misunderstanding, the meanings of certain termswill be specifically defined or clarified.

The terms “MXenes” or “two-dimensional (2D) crystalline transition metalcarbides” or two-dimensional (2D) transition metal carbides” may be usedinterchangeably to refer collectively to compositions described hereinas comprising substantially two-dimensional crystal lattices of thegeneral formulae M_(n+1)X_(n)(T_(s)), M₂A₂X(T_(s)), andM′₂M″_(n)X_(n+1)(T_(s)), where M, M′, M″, A, X, and T_(s) are definedherein. Supplementing the descriptions herein, M_(n+1)X_(n)(T_(s))(including M′₂M″_(m)X_(m+1)+(T_(s)) compositions) may be viewed ascomprising free standing and stacked assemblies of two dimensionalcrystalline solids. Collectively, such compositions are referred toherein as “M_(n+1)X_(n)(T_(s)),” “MXene,” “MXene compositions,” or“MXene materials.” Additionally, these terms “M_(n+1)X_(n)(T_(s)),”“MXene,” “MXene compositions,” or “MXene materials” can alsoindependently refer to those compositions derived by the chemicalexfoliation of MAX phase materials, whether these compositions arepresent as free-standing 2-dimensional or stacked assemblies (asdescribed further below). These compositions may be comprised ofindividual or a plurality of such layers. In some embodiments, theMXenes comprising stacked assemblies may be capable of, or have atoms,ions, or molecules, that are intercalated between at least some of thelayers. In other embodiments, these atoms or ions are lithium.

The term “crystalline compositions comprising at least one layer havingfirst and second surfaces, each layer comprising a substantiallytwo-dimensional array of crystal cells” refers to the unique characterof these materials. For purposes of visualization, the two-dimensionalarray of crystal cells may be viewed as an array of cells extending inan x-y plane, with the z-axis defining the thickness of the composition,without any restrictions as to the absolute orientation of that plane oraxes. It is preferred that the at least one layer having first andsecond surfaces contain but a single two-dimensional array of crystalcells (that is, the z-dimension is defined by the dimension ofapproximately one crystal cell), such that the planar surfaces of saidcell array defines the surface of the layer; it should be appreciatedthat real compositions may contain portions having more than singlecrystal cell thicknesses.

That is, as used herein, “a substantially two-dimensional array ofcrystal cells” refers to an array which preferably includes a lateral(in x-y dimension) array of crystals having a thickness of a single unitcell, such that the top and bottom surfaces of the array are availablefor chemical modification.

The MXene component of these compositions can be any of the compositionsdescribed in any one of U.S. patent application Ser. No. 14/094,966,International Applications PCT/US2012/043273, PCT/US2013/072733,PCT/US2015/051588, PCT/US2016/020216, or PCT/US2016/028,354. Specificsuch compositions are described elsewhere herein. In certain preferredembodiments, the MXenes comprise substantially two-dimensional array ofcrystal cells, each crystal cell having an empirical formula ofM_(n+1)X_(n), or M′₂M″_(n)X_(n+1), where M, M′, M″, and X are definedelsewhere herein. Those descriptions are incorporated here. In someindependent embodiments, M is Ti or Ta.

MXenes are known in the art to include nanosheet compositions comprisingsubstantially two-dimensional array of crystal cells having the generalformulae M₂X, M₃X₂ and M₄X₃. The MXene compositions described herein arealso sometimes described in terms of the phrase “MX-enes” or “MX-enecompositions.” MXenes have shown great promise for a variety ofapplications including energy storage, electromagnetic interferenceshielding, sensors, water purifications, and medicine.

In some embodiments, MXenes are described as two-dimensional transitionmetal carbides, nitrides, or carbonitrides comprising at least one layerhaving first and second surfaces, each layer described by a formulaM_(n+1)X_(n)T_(x) and comprising:

-   -   a substantially two-dimensional array of crystal cells,    -   each crystal cell having an empirical formula of M_(n+1)X_(n),        such that each X is positioned within an octahedral array of M,    -   wherein M is at least one Group IIIB, IVB, VB, or VIB metal,    -   wherein each X is C, N, or a combination thereof;    -   n=1, 2, or 3; and wherein    -   T_(x) represents surface termination groups.

These so-called MXene compositions have been described in U.S. Pat. No.9,193,595 and Application PCT/US2015/051588, filed Sep. 23, 2015, eachof which is incorporated by reference herein in its entirety at leastfor its teaching of these compositions, their (electrical) properties,and their methods of making. That is, any such composition described inthis Patent is considered as applicable for use in the presentapplications and methods and within the scope of the present invention.For the sake of completeness, M can be at least one of Sc, Y, Lu, Ti,Zr, Hf, V, Nb, Ta, Cr, Mo, or W. In certain embodiments in this class, Mis at least one Group IVB, Group VB, or Group VIB metal, preferably Ti,Mo, Nb, V, or Ta. Certain of these compositions include those having oneor more empirical formula wherein M_(n+1)X_(n) comprises Sc₂C, Ti₂C,V₂C, Cr₂C, Cr₂N, Zr₂C, Nb₂C, Hf₂C, Ti₃C₂, V₃C₂, Ta₃C₂, Ti₄C₃, V₄C₃,Ta₄C₃, Sc₂N, Ti₂N, V₂N, Cr₂N, Cr₂N, Zr₂N, Nb₂N, Hf₂C, Ti₃N₂, V₃C₂,Ta₃C₂, Ti₄N₃, V₄C₃, Ta₄N₃ or a combination or mixture thereof. Inparticular embodiments, the M_(n+1)X_(n) structure comprises Ti₃C₂,Ti₂C, Ta₄C₃ or (V_(1/2)Cr_(1/2))₃C₃. In some embodiments, M is Ti or Ta,and n is 1, 2, or 3, for example having an empirical formula Ti₃C₂ orTi₂C. In some of these embodiments, at least one of said surfaces ofeach layer has surface terminations comprising hydroxide, oxide,sub-oxide, or a combination thereof. In certain preferred embodiments,the MXene composition is described by a formula M_(n+1)X_(n)T_(x), whereM_(n+1)X_(n) are Ti₂CT_(x), Mo₂TiC₂T_(x), Ti₃C₂T_(x), or a combinationthereof, and T_(x) is as described herein. Those embodiments wherein Mis Ti, and n is 1 or 2, preferably 2, are especially preferred.

Additionally, or alternatively, the articles of manufacture and methodsuse compositions, wherein the two-dimensional transition metal carbide,nitrides, or carbon nitride comprises a composition having at least onelayer having first and second surfaces, each layer comprising:

-   -   a substantially two-dimensional array of crystal cells,    -   each crystal cell having an empirical formula of        M′₂M″_(n)X_(n+1), such that each X is positioned within an        octahedral array of M′ and M″, and where M″_(n) are present as        individual two-dimensional array of atoms intercalated        (sandwiched) between a pair of two-dimensional arrays of        M′atoms,    -   wherein M′ and M″ are different Group IIIB, IVB, VB, or VIB        metals (especially where M′ and M″ are Ti, V, Nb, Ta, Cr, Mo, or        a combination thereof),    -   wherein each X is C, N, or a combination thereof, preferably C;        and    -   n=1 or 2.

These compositions are described in greater detail in ApplicationPCT/US2016/028354, filed Apr. 20, 2016, which is incorporated byreference herein in its entirety at least for its teaching of thesecompositions and their methods of making. For the sake of completeness,in some embodiments, M′ is Mo, and M″ is Nb, Ta, Ti, or V, or acombination thereof. In other embodiments, n is 2, M′ is Mo, Ti, V, or acombination thereof, and M″ is Cr, Nb, Ta, Ti, or V, or a combinationthereof. In still further embodiments, the empirical formulaM′₂M″_(n)X_(n+1) comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃,Cr₂TiC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, or V₂TiC₂,preferably Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, or Mo₂NbC₂, or their nitride orcarbonitride analogs. In still other embodiments, M′₂M″_(n)X_(n+1)comprises Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Cr₂Ti₂C₃, Cr₂V₂C₃,Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃, V₂Ta₂C₃, V₂Nb₂C₃, orV₂Ti₂C₃, preferably Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Ti₂Nb₂C₃,Ti₂Ta₂C₃, or V₂Ta₂C₃, or their nitride or carbonitride analogs.

Each of these compositions having empirical crystalline formulaeM_(n+1)X_(n) or M′₂M″_(n)X_(n+1) are described in terms of comprising atleast one layer having first and second surfaces, each layer comprisinga substantially two-dimensional array of crystal cells. In someembodiments, these compositions comprise layers of individualtwo-dimensional cells. In other embodiments, the compositions comprise aplurality of stacked layers. Additionally, in some embodiments, at leastone of said surfaces of each layer has surface terminations (optionallydesignated “T_(s)” or “T_(x)” or “T_(z)”) comprising alkoxide,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, thiol, or a combination thereof. In someembodiments, at least one of said surfaces of each layer has surfaceterminations comprising alkoxide, fluoride, hydroxide, oxide, sub-oxide,or a combination thereof. In still other embodiments, both surfaces ofeach layer have said surface terminations comprising alkoxide, fluoride,hydroxide, oxide, sub-oxide, or a combination thereof. As used hereinthe terms “sub-oxide,” “sub-nitride,” or “sub-sulfide” is intended toconnote a composition containing an amount reflecting asub-stoichiometric or a mixed oxidation state of the M metal at thesurface of oxide, nitride, or sulfide, respectively. For example,various forms of titania are known to exist as TiO_(x), where x can beless than 2. Accordingly, the surfaces of the present invention may alsocontain oxides, nitrides, or sulfides in similar sub-stoichiometric ormixed oxidation state amounts.

In the present disclosure, these MXenes may comprise simple individuallayers, a plurality of stacked layers, or a combination thereof. Eachlayer may independently comprise surfaces functionalized by any of thesurface coating features described herein (e.g., as in alkoxide,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, thiol, or a combination thereof) or may be alsopartially or completely functionalized by polymers, either on thesurface of individual layers, for example, where the two-dimensionalcompositions are embedded within a polymer matrix, or the polymers maybe intercalated between layers to form structural composites, or both.

In certain applications, the MXene surface coatings can be adjusted torange from hydrophobic to hydrophilic, depending on post-synthesistreatment regimes.

The terms “MXenes” or “two-dimensional (2D) crystalline transition metalcarbides” or two-dimensional (2D) transition metal carbides” may be usedinterchangeably to refer collectively to compositions described hereinas comprising substantially two-dimensional crystal lattices of thegeneral formulae M_(n+1)X_(n)(T_(s)), M₂A₂X(T_(s)), andM′₂M″_(n)X_(n+1)(T_(s)), where M, M′, M″, A, X, and T_(s) are definedherein. Supplementing the descriptions herein, M_(n+1)X_(n)(T_(s))(including M′₂M″_(m)X_(m+1)+(T_(s)) compositions) may be viewed ascomprising free standing and stacked assemblies of two dimensionalcrystalline solids. Collectively, such compositions are referred toherein as “M_(n+1)X_(n)(T_(s)),” “MXene,” “MXene compositions,” or“MXene materials.” Additionally, these terms “M_(n+1)X_(n)(T_(s)),”“MXene,” “MXene compositions,” or “MXene materials” can alsoindependently refer to those compositions derived by the chemicalexfoliation of MAX phase materials, whether these compositions arepresent as free-standing 2-dimensional or stacked assemblies (asdescribed further below). These compositions may be comprised ofindividual or a plurality of such layers. In some embodiments, theMXenes comprising stacked assemblies may be capable of, or have atoms,ions, or molecules, that are intercalated between at least some of thelayers. In other embodiments, these atoms or ions are lithium.

The term “crystalline compositions comprising at least one layer havingfirst and second surfaces, each layer comprising a substantiallytwo-dimensional array of crystal cells” refers to the unique characterof these materials. For purposes of visualization, the two-dimensionalarray of crystal cells may be viewed as an array of cells extending inan x-y plane, with the z-axis defining the thickness of the composition,without any restrictions as to the absolute orientation of that plane oraxes. It is preferred that the at least one layer having first andsecond surfaces contain but a single two-dimensional array of crystalcells (that is, the z-dimension is defined by the dimension ofapproximately one crystal cell), such that the planar surfaces of saidcell array defines the surface of the layer; it should be appreciatedthat real compositions may contain portions having more than singlecrystal cell thicknesses.

That is, as used herein, “a substantially two-dimensional array ofcrystal cells” refers to an array which preferably includes a lateral(in x-y dimension) array of crystals having a thickness of a single unitcell, such that the top and bottom surfaces of the array are availablefor chemical modification.

It was determined that similar to clays, MXene surfaces are negativelycharged while their edges are positively charged. Thus, as shown in FIG.1 , a working hypothesis—confirmed herein—was that if the polyanionsadsorb at the MXene edges they may reduce their propensity foroxidation. MXenes generally tend to oxidize when stored as colloid inwater at ambient conditions. But by capping the edges of individualMXene flakes by polyanions significantly reduces the rate of oxidationand makes long term storage of MXene as possible. This is importantbecause helps to make solution processing of MXene scalable and facile.

EMBODIMENTS

The following listing of Embodiments is illustrative only and does notserve to limit the scope of the present disclosure or the appendedclaims.

Embodiment 1. A method of stabilizing MXene compositions againstoxidation, comprising: contacting a MXene composition with a polyanionicsalt, the contacting being performed so as to give rise to acorresponding polyanionic salt-stabilized MXene composition.

The MXene composition can, when contacted with the polyanionic salt, bein water or other aqueous medium. The salt-stabilized MXene compositioncan be dispersed in water (or other aqueous medium). Alternatively, thesalt-stabilized MXene composition can also be dry or be essentially freeof water.

The MXene composition that is contacted with the polyanionic salt can beaqueous, e.g., present in water or other aqueous medium. The MXenecomposition can also be present as a colloid or as another suspendedform, though this is not a requirement.

The polyanionic salt can be present at, e.g., from about 0.01 to about10 M, from about 0.1 to about 8 M, from about 0.01 to about 5 M, fromabout 0.1 to about 3 M, or even from about 0.1 to about 1 M. Asdescribed elsewhere herein (but without being bound to any particulartheory), the polyanion component of the polyanionic salt associates withthe edge of a MXene sheet by way of interaction between the positivecharge at the edge of the sheet and the negative charge of thepolyanion.

Embodiment 2. The method of Embodiment 1, wherein the MXene compositioncomprises: (a) at least one layer having first and second surfaces andan edge, each layer comprising a substantially two-dimensional array ofcrystal cells, each crystal cell having an empirical formula ofM_(n+1)X_(n), such that each X is positioned within an octahedral arrayof M, wherein M is at least one Group IIIB, IVB, VB, or VIB metal,wherein each X is C, N, or a combination thereof; n=1, 2 or 3; andwherein at least one of said surfaces of each layer has surfaceterminations comprising alkoxide, carboxylate, halide, hydroxide,hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or acombination thereof, or (b) at least one layer having first and secondsurfaces and an edge, each layer comprising: a substantiallytwo-dimensional array of crystal cells, each crystal cell having anempirical formula of M′₂M″_(n)X_(n+1), such that each X is positionedwithin an octahedral array of M′ and M″, and where M″_(n) are present asindividual two-dimensional array of atoms intercalated (sandwiched)between a pair of two-dimensional arrays of M′atoms, wherein M′ and M″are different Group IIIB, IVB, VB, or VIB metals, wherein each X is C,N, or a combination thereof, preferably C; and n=1 or 2.

Embodiment 3. The method of Embodiment 1, wherein M_(n+1)X_(n) comprisesSc₂C, Sc₂N, Ti₂C, Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N,Hf₂C, Hf₂N, Ti₃C₂, Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃,Ta₄C₃, Ta₄N₃, or a combination thereof.

Embodiment 4. The method of Embodiment 1, wherein M_(n+1)X_(n) comprisesTi₃C₂, Ti₃CN, Ti₂C, Ta₄C₃ or (V_(1/2)Cr_(1/2))₃C₂.

Embodiment 5. The method of Embodiment 1, wherein M is at least oneGroup IVB, Group VB, or Group VIB metal.

Embodiment 6 The method of Embodiment 1, wherein M is at least one ofSc, Y, Lu, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W.

Embodiment 7. The method of Embodiment 1, wherein M′₂M″_(n)X_(n+1)comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃, Cr₂TiC₂, Cr₂VC₂,Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, or V₂TiC₂, preferablyMo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃,Mo₂Ta₂C₃, Cr₂Ti₂C₃, Cr₂V₂C₃, Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃,Ti₂Ta₂C₃, V₂Ta₂C₃, V₂Nb₂C₃, or V₂Ti₂C₃, preferably Mo₂Ti₂C₃, Mo₂V₂C₃,Mo₂Nb₂C₃, Mo₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃, or V₂Ta₂C₃, or their nitride orcarbonitride analogs.

Embodiment 8. The method of any one of Embodiments 1-7, wherein thepolyanionic salt comprises at least one of a polyphosphate, apolyborate, and a polysilicate.

Embodiment 9. The method of Embodiment 8, wherein the polyphosphatecomprises diphosphate, triphosphate, tetraphosphate, or any combinationthereof.

Embodiment 10. The method of Embodiment 8, wherein the polyboratecomprises diborate, triborate, tetraborate, pentaborate, or anycombination thereof.

Embodiment 11. The method of Embodiment 8, wherein the polysilicatecomprises orthosilicate, disilicate, metasilicate, or pyrosilicate.

Embodiment 12. The method of Embodiment 8, wherein the polysilicatecomprises a sorosilicate, a cyclosilicate, a single-chain inosilicate, adouble-chain inosilicate, a phyllosilicate, or a tectosilicate.

Embodiment 13. The method of any one of Embodiments 1-12, wherein thepolyanionic salt comprises an alkali metal or an alkaline earth metal.

Embodiment 14. The method of Embodiment 13, wherein the polyanionic saltcomprises an alkali metal.

Embodiment 15. The method of any one of Embodiments 1-14, whereinpolyanions of the polyanionic salt are associated with the edge of theMXene composition.

Embodiment 16. The method of any one of Embodiments 1-15, wherein theMXene composition is in colloid form. The salt-stabilize MXenecomposition can also be in colloid form.

Embodiment 17. A polyanionic salt-stabilized MXene composition derivedor derivable from a method of any one of Embodiments 1-16.

Embodiment 18. A composition, comprising: a (salt-stabilized) MXenecomposition having at least one layer having first and second surfacesand an edge, at least one polyanion associated with the edge.

As described elsewhere herein, the salt-stabilized composition can beaqueous; e.g., the MXene composition can be dispersed in water or otheraqueous solvent. The salt-stabilized composition can be free-standing.The salt-stabilized composition can be present in platelet form, e.g.,as nanosheets.

The salt-stabilized MXene composition can be dispersed in a polymer orother matrix material. In this way, one can form composite materialsthat include the disclosed salt-stabilized MXene compositions.

It should be understood that the disclosed compositions (and methods)can include one polyanion or multiple polyanions. As an example, acomposition can include a polyphosphate polyanion and a polysilicatepolyanion. As another example, a composition can include a firstpolyphosphate polyanion and a second polyphosphate polyanion thatdiffers from the first polyphosphate polyanion.

Embodiment 19. The composition of Embodiment 18, wherein the MXenecomposition comprises: (a) at least one layer having first and secondsurfaces and an edge, each layer comprising a substantiallytwo-dimensional array of crystal cells, each crystal cell having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M, wherein M is at least one Group IIIB, IVB, VB,or VIB metal, wherein each X is C, N, or a combination thereof; n=1, 2or 3; and wherein at least one of said surfaces of each layer hassurface terminations comprising alkoxide, carboxylate, halide,hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,thiol, or a combination thereof, or (b) at least one layer having firstand second surfaces and an edge, each layer comprising: a substantiallytwo-dimensional array of crystal cells, each crystal cell having anempirical formula of M′₂M″_(n)X_(n+1), such that each X is positionedwithin an octahedral array of M′ and M″, and where M″_(n) are present asindividual two-dimensional array of atoms intercalated (sandwiched)between a pair of two-dimensional arrays of M′atoms, wherein M′ and M″are different Group IIIB, IVB, VB, or VIB metals, wherein each X is C,N, or a combination thereof, preferably C; and n=1 or 2.

Embodiment 20. The composition of Embodiment 18, wherein the polyanionis a polyborate, a polyphosphate, or a polysilicate.

Embodiment 21. The composition of Embodiment 18, wherein the polyanionis a polyphosphate.

Embodiment 22. An electronic device comprising the composition of anyone of Embodiments 18-21.

EXAMPLES

The Examples described herein are provided to illustrate some of theconcepts described within this disclosure. While each Example isconsidered to provide specific individual embodiments of composition,methods of preparation and use, none of the Examples should beconsidered to limit the more general embodiments described herein. Inparticular, while the examples provided here focus on specific MXenematerials, it is believed that the principles described are relevant toother such MXene materials. Accordingly, the descriptions provided hereshould not be construed to limit the disclosure, and the reader isadvised to look to the nature of the claims as a broader description.

In the following examples, efforts have been made to ensure accuracywith respect to numbers used (e.g. amounts, temperature, etc.) but someexperimental error and deviation should be accounted for. Unlessindicated otherwise, temperature is in degrees C., pressure is at ornear atmospheric.

Experimental Details

Two batches of aqueous Ti₃C₂T_(z) and V₂CT_(z) colloidal suspensionswere produced according to the details described herein.

The Ti₃C₂T_(z) colloid batch was divided into 11 batches of 50 ml each.Further, three aliquots of the above salts were made such that the finalsalt concentration in the 50 ml MXene colloid were 0.1 M, 0.3 M or 0.5M. The remaining suspensions were used as controls, with no salt added.All the samples were kept open to air for a month before the sampleswere characterized for oxidation.

In this disclosure, the suffix P is used after the salt molar ratios toindicate the addition of polyphosphate salt, while the suffixes B and Siindicate the addition of polyborate and polysilicate salts,respectively. The Ti and V suffixes, in turn refer to the Ti₃C₂T_(z) andV₂CT_(z) MXenes, respectively. For example, the 3 different molar ratiosmentioned above for the polyphosphate salts when mixed with Ti₃C₂T_(z)suspension will be referred to as 0.1PTi, 0.3PTi and 0.5PTi.

Two control samples were also characterized; one in which no salt wasadded, labelled 0Ti, the other fresh MXene collected right after etchingand washing—i.e. not allowed to oxidize—labelled FTi.

For the V₂CT_(z) composition two controls were used: One in which theV₂CT_(z) was held in a water open to the atmosphere for 3 weeks labeledas 0V. The other was a MXene film kept under vacuum (≈0.04 atm) also for3 weeks labeled FV. For V₂CT_(z) colloidal suspensions, only one samplewith 0.1 M polyphosphate salt was made and characterized. Thisconcentration was chosen because, as shown below, it was sufficient tostop oxidation in Ti₃C₂T_(z).

Example Synthesis

The Ti₃AlC₂ MAX powders were made by mixing titanium carbide, TiC, (AlfaAesar, 99.5% 2 μm), aluminum (Alfa Aesar, 99.5%, −325 mesh), and Ti(Alfa Aesar, 99.5%, −325 mesh), powders in a molar ratio of 2:1.05:1,respectively. The mixed powders were ball milled for 24 h at 70 rpm thenheated under flowing argon (Ar) at 1450° C. for 2 h. The heating andcooling rates were set at 5° C./min.

The resulting loosely sintered blocks were ground using a milling bit ona drill press. The milled powders were passed through a 400 mesh(particle size <38 μm) sieve for further experiments.

The sieved powder was etched in a LiF and HCl solution. First, 5 g ofLiF (Alfa Aesar, 99.5%, 325 mesh) was dissolved in 50 mL of 12 M HCl(Fisher Scientific), after which 5 g of the Ti₃AlC₂ powder was slowlyadded to the solution. The latter was stirred for 24 h at 35° C. and 500rpm, and the slurry was later transferred into a 50 mL centrifuge tubeand DI water was added to completely fill the remaining volume. It wasthen centrifuged at 3500 rpm for 120 s and the resulting clearsupernatant was discarded and the washing was repeated several timesuntil the pH of the solution was 7, at which point deionized, DI, waterwas added to the resulting Ti₃C₂T_(z) “clay” and the mixture sonicatedunder bubbling Ar flow for 1 h. Ice was added to the sonication bath toavoid oxidation. The solution was then centrifuged for 1 h at 3500 rpmand the supernatant was collected for further use.

The solid content of the supernatant was determined by vacuum filteringa known solution volume and measuring the weight of the resulting freestanding MXene film upon drying in a vacuum oven at 100° C. overnight,and found to be roughly 15 mg/ml.

The colloid was separated into 11 parts of 50 ml each. In 3 parts sodiumpolyphosphate (Sodium polyhposphate, Acros Organics) salt was added toobtain final concentrations of 0.1 M, 0.3 M and 0.5 M. Similarly, sodiumpolyborate (sodium tertraborate decahydrate, Alfa Aesar) and sodiumpolysilicate (sodium metasilicate, Alfa Aesar) were added to other 50 mlMXene colloidal suspensions.

After brief stirring, the samples were kept open to air but undisturbedfor 1 month. Occasionally DI water added to the containers to compensatefor evaporation. In other words, a total volume of 50 ml was alwaysmaintained. One part of the sample was filtered using vacuum filtrationright after synthesis for characterization of pristine MXene samples.Another part was stored without the polyanionic salts, which acted as acontrol sample.

After a month all the salt containing samples were centrifuged at 5000rpm for 120 s and the sediment was collected. After the sediments arecollected, they were further washed with DI water 3 more times usingsimilar washing procedure described above. After 3 washes the MXene wereeasily dispersed back into the DI water, forming a colloid just byshaking. This colloid was in turn vacuum filtered and the free-standingfilms obtained were later powdered using a mortar and pestle and usedfor characterization like XRD, XPS and SEM.

To prepare TEM a drop of colloid from the respective sample was cast ona lacy carbon coated copper grid (Cu-400LC, Pacific Grid-Tech) and driedunder vacuum.

The V₂AlC MAX powders were made by mixing vanadium carbide, VC, (AlfaAesar, 99.5% 2 microns), Al (Alfa Aesar, 99.5%, −325 mesh), and V (AlfaAesar, 99.5%, −325 mesh), powders in a molar ratio of 1:1.13:1,respectively. The mixed powders were ball milled for 24 h at 70 rpm thenheated under flowing Ar at 1500° C. for 4 h. The heating and coolingrates were set at 5° C./min.

The resulting loosely sintered blocks were ground using a milling bit ona drill press. The milled powders were passed through a 400 mesh(particle size <38 μm) sieve for further experiments.

Three grams of the sieved powders were etched by adding them to 30 ml ofa 50% HF solution (Acros Organics). The solution was held at 55° C. for3 d and stirred at 500 rpm. Later it was washed until pH 7 using asimilar process discussed above.

To delaminate the V₂CT_(z) multilayer, 3 ml of 1.5 M tertbutyl ammoniumhydroxide (TBAOH) was added to added to the ML MXene obtained from abovestep and mixed using a vortex mixer for 0.5 h, after which the solutionwas washed with DI water once followed by 3 washes with ethanol.Finally, the ethanol was decanted and 100 ml DI water is added to theTBA intercalated multilayers, and the mixture was further mixed using avortex shaker for 0.5 h. This disperses the MXene in water and theexfoliated MXene is separated from the colloid by centrifuging it at3500 rpm for 0.5 h.

After centrifuging the MXene colloid is divided into 2 parts of 50 mleach, to one-part sodium polyphosphate salt is added such that the finalconcentration if 0.1 M. Nothing is added to the other part. Both thevials are then left open to air for 3 weeks.

Characterization

A SEM (Zeiss Supra 50VP, Germany) was used to examine the morphology andobtain micrographs of the samples.

XRD patterns were recorded using a X-Ray diffractometer (Rigaku SmartLab, Tokyo, Japan) using Cu K_(α) radiation (40 kV and 40 mA) with astep size of 0.02° and dwell time of 1 s, in the 2-65° 2θ range.

Monochromatic Al-Kα X-rays with the spot size of 200 μm was used toirradiate the sample surface. Pass energy of 23.5 eV, with a step sizeof 0.5 eV was used to gather the high-resolution spectra. CasaXPSVersion 2.3.19PR1.0 software was used for analysis of spectra. The XPSspectra were calibrated by setting the valance edge to zero, which wascalculated by fitting the valence edge with a step down function andsetting the intersection to 0 eV.

Results and Discussion:

After storing in water for a month in case of Ti₃C₂T_(z), and 3 weeksfor V₂CT_(z) in an open container, the various colloid suspensions werewashed using DI water several times and filtered through a vacuum filterover a Celgard membrane and dried. The MXene films obtained were thencrushed using a mortar and pestle and used for further analysis.

FIG. 6A plots the X-ray diffraction (XRD) patterns of the 0.1PTi (blue,top), 0.1SiTi (green, 2^(nd) from top), 0.1BTi (red, 3^(rd) from top)and 0Ti (black, bottom) crushed films. The XRD patterns are plotted on alog scale along the y-axis to better visualize the minority phase peaksthat may be present.

From these results it is obvious that only 0Ti pattern (bottom patternin FIG. 6A) showed clear peaks belonging to rutile, TiO₂, as indicatedby asterisks (JCPDS-#12-1276). More importantly, these peaks are absentfrom all other samples, even those with the minimum salt concentrationof 0.1 M, regardless of their chemistry. The presence of the (002) peakaround 6° 2θ and the (110) peak around 61° 2θ in all 4 XRD patternsconfirm the presence of MXene and shows that phase degradation does notoccur during storage.

The very slight differences in the position of (002) peaks are possiblydue to varying levels of hydration in the interlayer space of theTi₃C₂T_(z) sheets. The broad peaks near 35° and 40° seen in XRD patternsof 0.1Si and 0.1B (FIG. 6A) are from a convolution of peaks representingthe (101) family of crystallographic planes and are typical ofTi₃C₂T_(z). Not surprisingly, the TiO₂ peaks were also absent in sampleswith higher polyanion salt concentrations (FIGS. 2A-2C).

Furthermore, the absence of peaks associated with the salts in FIG. 6Aand FIGS. 2A-2C suggests that these salts can be readily washed away,irrespective of their concentration. This is important in general sincesuch salts, if present, could be detrimental to some properties.

Because the V₂CT_(z) flakes in the colloidal suspension in which no saltwas added completely dissolved (see inset ii in FIG. 6B,) no diffractionpatterns could be obtained from this sample. On the other hand, the0.1PV and the FV (FIG. 6B) samples show the (002) and (110) peaks near6° and 64° respectively, that are typical of V₂CT_(z). No other peakswere observed, implying that, at least at the XRD level, the oxidationwas undetectable in both cases.

Probably the most dramatic evidence for the potency of the polyphosphateanions in slowing down oxidation are the photographs shown in the insetof FIG. 6B. The picture on the left (FIG. 6B(i)) is of the colloidalsuspension of the 0.1PV sample, that on the right (FIG. 6B(ii)) of asuspension with no polyphosphates, both stored in open container in airfor 3 weeks.

The V₂CT_(z) suspension without the salt not only oxidizes butcompletely dissolves giving the solution an orange tinge characteristicof V⁺⁵ ions. The 0.1PV sample, on the other hand, did not significantlychange color compared to a fresh colloidal suspension (not shown). Wenote in passing that the water remaining after making the 0.1PV filteredfilm was colorless, further confirming that the absence of any oxidepeaks in the XRD is not because these oxides dissolved in water.

Scanning electron microscope (SEM) micrographs for the 0PTi, 0.1PTi,0.1BTi, 0.1SiTi films are shown in FIGS. 7A-7D, respectively. Similarto, and consistent with, the XRD results, only in the 0PTi micrograph(FIG. 7A) is there evidence for oxidation in the form of a multitude ofnanometer sized particles. These particles appear brighter than theMXene flakes and based on the XRD patterns are most probably TiO₂nanoparticles. These particles are absent in the other samples. It isnoteworthy—and again in total accord with the XRD diffraction results,—that no salt crystals are seen in the SEM micrographs, furtherconfirming that the salts were washed off.

Apart from the titania nanoparticles, the planar morphology seen in all4 micrographs in FIG. 7A-7D are typical of vacuum filtered MXene filmsin which the 2D MXene sheets stack densely on top of each other. Themicrographs of FV samples also show nanometer sized particles over thesurface, which are possibly oxide particles (FIG. 3A). Without beingbound to any particular theory, the reason they do not result in XRDpeaks is most probably because of their poor crystallinity and/or lowconcentration. These particles are not seen in SEM micrographs of the0.1PV sample (FIG. 3B).

TEM was also used to assess the extent of oxidation on single MXeneflakes. The TEM micrographs showed that spindle shaped oxidenanoparticles only grew on Ti₃C₂T_(z) flakes that we stored without thepolyanions (FIG. 8A). In the other 3 samples (FIGS. 8B-8D) theseparticles were not seen. These results, yet again, directly demonstratethe effectiveness of our polyanion salt treatments in curtailingoxidation. The total lack of any particles in FIGS. 8B and 8C isnoteworthy.

To further probe the state of oxidation on the Ti₃C₂T_(z) flakes, X-rayphotoelectron (XPS) spectra were acquired. FIG. 9A plots the XPS spectraof 0.1PTi, 0.1BTi, 0.1SiTi, 0PTi and FTi samples. The two main peaksobserved in the Ti photoemission spectra of all 5 samples around 556 eVand 563 eV correspond to Ti 2p_(3/2) and Ti 2p_(1/2) peaks of Ti₃C₂T_(z)respectively. These peaks are a convolution of peaks ascribed to varioussurface terminations like —O, —OH and —F attached to the MXene surfaces.

The gray band, around 459 eV, shown on FIG. 9A, highlights the bindingenergies associated with Ti in a +4 oxidation state, viz TiO₂. The onlyspectrum for which a signal is obtained in that range is the sample inwhich no salts were added again confirming the important conclusion thatthese salts indeed are potent in arresting, or significantly mitigating,oxidation even under the most extreme conditions, viz. water in which Ois dissolved. Note that because the TiO₂ peaks are clearly visible evenwithout deconvolution of the Ti photoemission spectra, peak fitting wasnot necessary for this study.

A careful perusal of the spectra shown in FIG. 9A, however, show thatfor the 0.1PTi and FTi samples the signal is significantly flatter inthe gray band that those for the 0.1BTi and 0.1SiTi samples. For thelatter, a shoulder appears near the right edge of the gray bandsuggesting the presence of Ti in a +3 oxidation state. It follows thatfor reasons that are not entirely clear the phosphate polyanions are themost effective in preventing the oxidation of Ti-based MXenes and arethus recommended.

The XPS spectra of 0.1SiTi sample is unique in that a clear shoulder atlower binding energy (453 eV) is present. Without being bound to anyparticular theory, this peak suggests a possible reduction of theTi₃C₂T_(z) flakes.

Moreover, analysis of the XPS spectra, showed that the Ti:F ratios forthe FTi, 0PTi, 0.1PTi, and 0.1BTi samples were 2.3, 3.0, 3.7, 2.7,respectively. It follows that with time, some of the F-terminations arereplaced by —O or —OH terminations. Surprisingly, no F was detected inthe 0.1SiTi samples, and more work needs to be done to analyze the MXenesurface modification caused by the polysilicate salt. FIGS. 4A-4C showthe Ti region photoemission spectra of the higher salt concentrationsamples all the spectra show no presence of Ti⁴⁺ and the spectraprofiles are similar to those of their 0.1 M counterparts.

FIG. 9B compares the XPS spectra of a FV (black) and 0.1PV (red)samples. The two peaks around 515 eV and 523 eV seen in FIG. 9B areascribed to V₂p_(3/2) and V₂p_(1/2) respectively. The grey bands around517 eV and 525 eV represent V in a +5 oxidation state. From theseresults it is clear that the areas under the peaks in the grey bands issignificantly smaller in 0.1PV sample compared to the vacuum storedfilm. Note here that the comparison was between a colloidal suspensionand a V₂CT_(z) film stored in vacuum.

Lastly, to confirm that the phosphate polyanions cap the MXene edges,electron energy loss spectroscopy (EELS) was performed on a singleTi₃C₂T_(z) flake (FIG. 10B) taken from the 0.1PTi sample. The signalsfor Ti, C and P were collected moving from vacuum to the MXene flakeaveraging over a total distance of 150 nm, and the edge position was setat 0 nm. The LP1 and LP2 graphs (FIGS. 10A, 10C) show that the P signalis at a maximum exactly at the edges, thereby confirming our hypothesis.The Ti and C signals are more or less constant from the edges inward,which is to be expected.

Further, EELS spectra collected at 3 spots—one over vacuum (S1) and oneon the MXene surface (S3)—showed no phosphorus signal. A clearphosphorus signal was obtained from the edge (S2) (FIG. 5 ) furtherconfirming that the polyanions decorate the edges.

Without being bound to any particular theory, one can conclude that thiscapping passivates the edges and prevents their edge-in oxidation. Theseresults are noteworthy for two reasons: First is their practicalimportance. Second they provide, for the first time, direct evidence foranion adsorption at the edges of a Ti₃C₂T_(z) flake.

CONCLUSIONS

In conclusion, the addition of inorganic polyanionic salts to aqueousMXene colloids significantly slows their oxidation. We attribute thereason to capping of the positively charged MXene sheet edges by thepolyanions. And while the three salts tested—polyphosphates, polyboratesand polysilicates—slowed down oxidation, XPS revealed that thepolyphosphate salts are best.

Further it was found that concentrations of only 0.1M were enough tosuppress MXene oxidation at least for 3 weeks in aerated water at roomtemperature. Another advantage of using polyphosphate salts is theirnon-toxicity, low cost and green credentials rendering them useful forlong term storage of MXene colloids, even on an industrial scale.

The following references may be useful in understanding the scope of thepresent disclosure.

-   F. Xia, H. Wang, Y. Jia, Nat. Commun. 2014, 5, 4458.-   K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V.    Khotkevich, S. V. Morozov, A. K. Geim, Proc. Natl. Acad. Sci. 2005,    102, 10451-10453.-   M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y.    Gogotsi, M. W. Barsoum, ACS Nano 2012, 6, 1322-1331.-   L. Verger, C. Xu, V. Natu, H.-M. Cheng, W. Ren, M. W. Barsoum, Curr.    Opin. Solid State Mater. Sci. 2019, DOI    10.1016/j.cossms.2019.02.001.-   Q. Li, Q. Zhou, L. Shi, Q. Chen, J. Wang, J. Mater. Chem. A 2019, 7,    4291-4312.-   G. Wang, R. Pandey, S. P. Karna, Wiley Interdiscip. Rev. Comput.    Mol. Sci. 2017, 7, e1280.-   M. Sokol, V. Natu, S. Kota, M. W. Barsoum, Trends Chem. 2019, DOI    10.1016/j.trechm.2019.02.016.-   M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L.    Hultman, Y. Gogotsi, M. W. Barsoum, Adv. Mater. 2011, 23, 4248-4253.-   M. Ghidiu, M. R. Lukatskaya, M.-Q. Zhao, Y. Gogotsi, M. W. Barsoum,    Nature 2014, DOI 10.1038/nature13970.-   M. Han, X. Yin, H. Wu, Z. Hou, C. Song, X. Li, L. Zhang, L. Cheng,    ACS Appl. Mater. Interfaces 2016, 8, 21011-21019.-   V. Natu, M. Clites, E. Pomerantseva, M. W. Barsoum, Mater. Res.    Lett. 2018, 6, 230-235.-   L. Ding, Y. Wei, L. Li, T. Zhang, H. Wang, J. Xue, L.-X. Ding, S.    Wang, J. Caro, Y. Gogotsi, Nat. Commun. 2018, 9, 155.-   C. E. Ren, M. Alhabeb, B. W. Byles, M.-Q. Zhao, B. Anasori, E.    Pomerantseva, K. A. Mahmoud, Y. Gogotsi, ACS Appl. Nano Mater. 2018,    1, 3644-3652.-   S. Intikhab, V. Natu, J. Li, Y. Li, Q. Tao, J. Rosen, M. W.    Barsoum, J. Snyder, J. Catal. 2019, 371, 325-332.-   O. Mashtalir, K. M. Cook, V. N. Mochalin, M. Crowe, M. W.    Barsoum, Y. Gogotsi, J. Mater. Chem. A 2014, 2, 14334-14338.-   C. J. Zhang, S. Pinilla, N. McEvoy, C. P. Cullen, B. Anasori, E.    Long, S.-H. Park, A. Seral-Ascaso, A. Shmeliov, D. Krishnan, et al.,    Chem. Mater. 2017, 29, 4848 4856.-   S. Huang, V. N. Mochalin, Inorg. Chem. 2019, 58, 1958-1966.-   K. Maleski, V. N. Mochalin, Y. Gogotsi, Chem. Mater. 2017, 29, 1632    1640.-   Y. Chae, S. J. Kim, S.-Y. Cho, J. Choi, K. Maleski, B.-J. Lee, H.-T.    Jung, Y. Gogotsi, Y. Lee, A. Chi Won, Nanoscale 2019, DOI    10.1039/C9NR00084D.-   J. A. Evans, A. M. Foster, J.-M. Huet, L. Reinholdt, K. Fikiin, C.    Zilio, M. Houska, A. Landfeld, C. Bond, M. Scheurs, et al., Energy    Build. 2014, 74, 141-151.-   J. I. Bidwell, W. B. Jepson, G. L. Toms, Clay Miner. 1970, 8,    445-459. J. K. Edzwald, D. C. Toensing, M. C.-Y. Leung, Environ.    Sci. Technol. 1976, 10, 485-490.-   V. Natu, M. Sokol, L. Verger, M. W. Barsoum, J. Phys. Chem. C 2018,    122, 27745-27753.-   M. Naguib, R. R. Unocic, B L. Armstrong, J. Nanda, Dalt. Trans.    2015, 44, 9353-9358.-   M. Ghidiu, M. W. Barsoum, J. Am. Ceram. Soc. 2017, DOI    10.1111/jace.15124.-   M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L.    Hultman, Y. Gogotsi, M. W. Barsoum, Adv. Mater. 2011, 23, 4248-4253.-   M. Ghidiu, J. Halim, S. Kota, D. Bish, Y. Gogotsi, M. W. Barsoum,    Chem. Mater. 2016, 28, 3507-3514.-   P. Collini, S. Kota, A. D. Dillon, M. W. Barsoum, A. T. Fafarman, J.    Electrochem. Soc. 2017, 164, D573-D580.-   I. Persson, L.-Å. Näslund, J. Halim, M. W. Barsoum, V.    Darakchieva, J. Palisaitis, J. Rosen, P. O. Å. Persson, 2D Mater.    2017, 5, 015002.-   J. Halim, K. M. Cook, M. Naguib, P. Eklund, Y. Gogotsi, J.    Rosen, M. W. Barsoum, Appl. Surf Sci. 2016, 362, 406-417.-   Y. Yoon, T. A. Le, A. P. Tiwari, I. Kim, M. W. Barsoum, H. Lee,    Nanoscale 2018, 10, 22429-22438.-   M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau, A. R.    Gerson, R. S. C. Smart, Appl. Surf Sci. 2011, 257, 2717-2730.-   T. V. Kulakovskaya, V. M. Vagabov, I. S. Kulaev, Process Biochem.    2012, 47, 1-10.-   V. Natu, M. Sokol, L. Verger, M. W. Barsoum, J. Phys. Chem. C 2018,    122, 27745-27753.-   G. Greczynski, L. Hultman, Chem Phys Chem 2017, 18, 1507-1512.

As those skilled in the art will appreciate, numerous modifications andvariations of the present invention are possible in light of theseteachings, and all such are contemplated hereby. All references citedwithin this specification are incorporated by reference in theirentireties for all purposes, or at least for their teachings in thecontext of their recitation.

1. A composition, comprising: a MXene composition having at least onelayer having first and second surfaces and an edge; at least onepolyanion, optionally wherein the at least one polyanion is associatedwith the edge of the MXene composition; and a medium, the MXenecomposition and the at least one polyanion being present in the medium.2. The composition of claim 1, wherein the MXene composition comprises:(a) at least one layer having first and second surfaces and an edge,each layer comprising a substantially two-dimensional array of crystalcells, each crystal cell having an empirical formula of M_(n+1)X_(n),such that each X is positioned within an octahedral array of M, whereinM is at least one Group IIIB, IVB, VB, or VIB metal, wherein each X isC, N, or a combination thereof; n=1, 2 or 3; and wherein at least one ofsaid first and second surfaces of each layer has surface terminationscomprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide,sub-oxide, nitride, sub-nitride, sulfide, thiol, or a combinationthereof, or (b) at least one layer having first and second surfaces andan edge, each layer comprising: a substantially two-dimensional array ofcrystal cells, each crystal cell having an empirical formula ofM′₂M″_(n)X_(n+1), such that each X is positioned within an octahedralarray of M′ and M″, and where M″_(n) are present as individualtwo-dimensional array of atoms intercalated (sandwiched) between a pairof two-dimensional arrays of M′atoms, wherein M′ and M″ are differentGroup IIIB, IVB, VB, or VIB metals, wherein each X is C, N, or acombination thereof, preferably C; and n=1 or
 2. 3. The composition ofclaim 1, wherein the polyanion is any one or more of a polyborate, apolyphosphate, or a polysilicate.
 4. The composition of claim 3, whereinthe polyanion is a polyborate.
 5. The composition of claim 4, whereinthe polyborate comprises any one or more of diborate, triborate,tetraborate, or pentaborate.
 6. The composition of claim 3, wherein thepolyanion is a polyphosphate.
 7. The composition of claim 6, wherein thepolyphosphate comprises any one or more of diphosphate, triphosphate, ortetraphosphate.
 8. The composition of claim 3, wherein the polyanion isa polysilicate.
 9. The composition of claim 8, wherein the polysilicatecomprises orthosilicate, disilicate, metasilicate, or pyrosilicate. 10.The composition of claim 8, wherein the polysilicate comprises asorosilicate, a cyclosilicate, a single-chain inosilicate, adouble-chain inosilicate, a phyllosilicate, or a tectosilicate.
 11. Thecomposition of claim 1, wherein the composition comprises an alkalimetal or an alkaline earth metal.
 12. The composition of claim 11,wherein the composition comprises an alkali metal.
 13. The compositionof claim 1, wherein the medium is water.
 14. The composition of claim13, wherein the water is aerated water.
 15. The composition of claim 1,wherein the medium is a polymer.
 16. The composition of claim 1, whereinthe MXene composition is essentially free of oxidation.
 17. A method ofstabilizing a MXene composition against oxidation, comprising:contacting (i) a polyanionic salt that comprises at least one of apolyphosphate, a polyborate, and a polysilicate, (ii) a MXenecomposition having at least one layer having first and second surfacesand an edge, and (iii) a medium, the contacting being performed so as togive rise to a corresponding MXene composition having at least one layerhaving first and second surfaces and an edge having associated therewiththe at least one of a polyborate, a polyphosphate, or a polysilicate.18. The method of claim 17, wherein the medium is water.
 19. The methodof claim 17, wherein the polyanionic salt comprises an alkali metal oran alkaline earth metal.
 20. The method of claim 19, wherein thepolyanionic salt comprises an alkali metal.